1. Field of the Invention
The invention relates to methods and devices for accurately monitoring the electrical current in electrolytic cells used to produce aluminum, other metals, or other chemical substances.
2. Description of the Related Art
Within industry there is an increasing need to accurately measure electrical currents and other electrical variables so as to control electrical equipment for the purposes of improving productivity or efficiency, enhancing safety and related objectives.
The production of many important materials and chemical substances such as aluminum, copper, zinc, chlorine and caustic soda is carried out within industry using electrolytic cells wherein a large direct electrical current (DC) is passed through the cell so as to bring about electrochemical reactions that produce the desired materials. In many cases the distribution of the electrical current within parts of the cell, or the variation of electrical current due to phenomena occurring in the cell, has an effect on process efficiency. In many cases the cost and difficulty of making measurements of electrical current and electrical current distribution within the cell preclude such measurements and opportunities for improvement of cell performance are unrealized.
Representative of these industries is the aluminum industry. The table below compares the electrical energy consumption of the world's aluminum industry with that of several countries. Clearly the electrical energy consumed by this industry is enormous and yet the efficiency of use of this energy is poor. Approximately 55% of the electrical energy entering an aluminum smelter becomes waste heat and only 45% is gainfully used to complete the necessary electrochemical reactions in electrochemical cells.
The industry also has a large carbon footprint. The production of each ton of aluminum by a cathodic reaction is accompanied by the production of 1.2 tons of CO2 by the anodic consumption of carbon anodes used in the process. Additional CO2, about 0.4 tons, is produced in the baking furnaces making the anodes. Furthermore, the cells suffer from periodic upsets called “anode effects” which cause emission of fluorinated hydrocarbons (PFCs) with pernicious global warming potential so that PFC emissions add approximately one ton of CO2 equivalents, bringing the total emissions to the order of 2.6 tons of CO2 equivalents per ton of aluminum. CO2 emissions at the power plants supplying the electricity might be included in any rational assessment of the carbon footprint of aluminum smelting. This would depend on the mix of power generation technology: zero for hydro, solar or nuclear, but up to 8 tons CO2 per ton of aluminum at coal burning plants. There are approximately 200 aluminum smelters around the world producing a total of approximately 40 million metric tons of metal per year. Consequently the worldwide carbon footprint is in the range 104-424 million metric tons CO2 equivalents per year. Thus, the ability to accurately monitor electrolytic cells could significantly reduce the CO2 output from aluminum production.
Electrical energy consumption (gigaWatt hours) 2007(source CIA Factbook)1United States3,717,0002China2,494,0003Japan946,3004Russia940,0005India587,900Aluminum Smelters548,8966Germany524,6007Canada522,4008France482,400
Determining an electrical current by measuring the magnetic field has been known since at least the beginning of the twentieth century. However, existing techniques for monitoring and measuring these magnetic fields are often inadequate for rapid, accurate monitoring.
It is known that one or more sensors may be used to determine the electrical current distribution in alumina reduction cells, commonly used in the production of aluminum, by measuring magnetic fields. Existing techniques may rely on measurements from differential signals from multiple sensors, or may rely on multiple sensor measurements to create an average measurement. However, these techniques have inaccuracies due to sensor calibration and temperature drift. In operation of a Hall-Héroult electrolytic cell anodes must be changed periodically. The used anode and anode rod must be removed and replaced with a new anode and anode rod. This necessarily effects the sensor calibration, and thus error is inevitably introduced whenever anodes are changed.
Existing techniques generally place multiple sensors adjacent to the conductor, potentially subjecting the sensors to changing and extreme temperatures. Extreme temperatures or temperature changes interferes with the sensors ability to provide accurate measurements.
Better measurement of process parameters (voltage, electrical current, temperature, etc.) leads to improvement in the performance of such production technologies whereby energy efficiency can be improved and carbon footprint reduced.
Thus, there exists a need to more accurately measure electrical fields in electrolytic cells by reducing the errors and inaccuracies in present-day measuring techniques. The present invention is an improved method of measuring large electrical currents found in such industries. The sensors used in the present invention are not in contact with the anode, and therefore provide consistent, accurate measurements even when anode changing is needed.